The present invention relates to the deposition of films, or layers, primarily in the fabrication of integrated circuits, but also in the manufacture of other products.
Integrated circuit fabrication procedures are composed of a variety of operations, including operations for depositing thin films on a semiconductor substrate, or wafer. Typically, a large number of identical integrated circuits are formed on such a wafer, which is then cut, or diced, into individual circuit chips.
Given the small dimensions of these integrated circuits, the quality of each deposited layer or film has a decisive influence on the quality of the resulting integrated circuit. Basically, the quality of a film is determined by its physical uniformity, including the uniformity of its thickness and its homogeneity.
In particular, several process steps require the ability to deposit high quality thin conductive films and to deposit conducting material in both high aspect ratio trenches and vias (and/or contacts).
According to the current state of the art, films, or layers, are deposited on a substrate according to two types of techniques: physical vapor deposition (PVD), which encompasses various forms of sputtering; and chemical vapor deposition (CVD). According to each type of procedure, a layer of material composed of a plurality of atoms or molecules of elements or compounds, commonly referred to collectively as xe2x80x9cadatomsxe2x80x9d, is deposited upon a substrate in a low pressure region.
In typical PVD procedures, a target material is sputtered to eject adatoms that then diffuse through the low pressure region and condense on the surface of the substrate on which the layer is to be deposited. This material forms a layer on the substrate surface. Continuation of this process leads to the growth of a thin film. The sputtering itself is a physical process which involves accelerating heavy ions from an ionized gas, such as argon, toward the target surface, where the ions act to dislodge and eject adatoms of the target material as a result of momentum exchange which occurs upon collision of the ions with the target surface.
On the other hand, in CVD procedures, two or more gases are introduced into a vacuum chamber where they react to form products. One of these products will be deposited as a layer on the substrate surface, while the other product or products are pumped out of the low pressure region.
Both types of deposition processes are advantageously performed with the assistance of a plasma created in the low pressure region. In the case of PVD processes, it is essential to provide a primary plasma to generate the ions that will be used to bombard the target. However, in these processes, a secondary plasma may be formed to assist the deposition process itself. In particular, a secondary plasma can serve to enhance the mobility of adatoms in proximity to the substrate surface.
Although CVD processes are widely used in the semiconductor fabrication industry, processes of this type have been found to possess certain disadvantages. For example, in order to employ CVD for a particular deposition operation, it is necessary to be able to create a chemical reaction that will produce, as one reaction product, the material to be deposited. In contrast, in theory, any material, including dielectric and conductive materials, can be deposited by PVD and this is the process of choice when deposition must be performed while maintaining the substrate temperature within predetermined limits, and particularly when deposition is to be performed while the substrate is at a relatively low temperature.
A film composed of a dielectric material can be formed by PVD either by directly sputtering a target made of the dielectric material, or by performing a reactive sputtering operation in which a conductive material is sputtered from a target and the sputtered conductive material then reacts with a selected gas to produce the dielectric material that is to be deposited. One exemplary target material utilized for direct sputtering is silicon dioxide. PVD can also be used for conductive layers.
The simplest known PVD structure has the form of a planar diode which consists of two parallel plate electrodes that define cathode constructed to serve as the target and an anode which supports the substrate. A plasma is maintained between the cathode and anode and electrons emitted from the cathode by ion bombardment enter the plasma as primary electrons and serve to maintain the plasma.
While a target made of a conductive material can be biased with a DC power supply, a target made of a dielectric material must be biased with high frequency, and particularly RF power, which can also assist the generation of ions in the plasma. The RF power is supplied to the target by a circuit arrangement including, for example, a blocking capacitor, in order to cause the applied RF power to result in the development of a DC self-bias on the target.
Since the planar diode configuration is not suitable for efficient generation of ions, DC and RF magnetron configurations have been developed for producing a magnetic field having field lines that extend approximately parallel to the target surface. This magnetic field confines electrons emitted from the target within a region neighboring the target surface, thereby improving ionization efficiency and the creation of higher plasma densities for a given plasma region pressure.
Additional configurations followed including the variety of cylindrical magnetrons. Several versions of the cylindrical magnetron variation have appeared in the patent prior art, in particular, the family of U.S. Pat. Nos. 4,132,613, 4,132,612, 4,116,794, 4,116,793, 4,111,782, 4,041,353, 3,995,187, 3,884,793 and 3,878,085.
As described in Thornton in xe2x80x9cInfluence of Apparatus Geometry and Deposition Conditions on the Structure and Topography of Thick Sputtered Coatingsxe2x80x9d, J. Vac. Sci. Technol., Vol 11, No 4, 666-670 (1974), the structure of a deposited metal film is dependent on both the temperature of the substrate and the gas pressure within the plasma region. The highest film quality can be achieved when the substrate is at a relatively low temperature and conditions are created to effect a certain level of bombardment of the substrate with ions from the plasma while the film is being formed. When optimum conditions are established, a dense, high quality thin film which is substantially free of voids and anomalies can be achieved.
It is known in the art that bombardment of the substrate with ions having energies under 200 eV, and preferably not greater than 30 eV, and more preferably between 10 and 30 eV, can result in the formation of dielectric films having optimum characteristics. This has been found to be true in the case of, for example, thin films of SiO2 and TiO2.
Achievement of high deposition rates and optimum quality of the deposited layer requires a high energy density in the plasma adjacent both the target and the substrate. Plasma energy flux (with units of J/m2-sec) is the product of the ion flux (with units of number of ions/m2-sec) and ion energy (with units of J/ion).
However, whereas the highest possible ion energy is desired adjacent the target to maximize the target sputtering rate, it has been found that the ion energy adjacent the substrate, i.e., the energy of ions that bombard the substrate, should be less than the maximum achievable for reasons relating primarily to layer quality.
For example, a reduced ion energy in the plasma adjacent the substrate reduces the rate of implantation of plasma gas ions into the substrate, as well damage to the substrate subsurface, and the creation of voids and mechanical stresses in the layer being formed.
Therefore, while it is desirable to have a high ion density in the plasma adjacent both the target and the substrate, different desiderata exist with respect to plasma ion energy. The plasma density in these systems are quite uniform, ie within +/xe2x88x9220% due to diffusion.
The plasma energy flux to the target and to the substrate are each limited by the ability to remove heat from the target or substrate/chuck, respectively. Moreover, the ion energy associated with ions striking the substrate surface is desired to be limited to a maximum value to assure layer deposition quality. Therefore, it is imperative to use a plasma source capable of generating a dense plasma ( greater than 1012 cmxe2x88x923) or, equivalently, a high ion current density at the target and substrate surfaces while enabling direct control of the ion energy via other means.
For PVD of conductive material, an electron cyclotron resonance (ECR) plasma source is known, capable of generating plasmas with a density in the range of 1012 cmxe2x88x923 and higher. However, the plasma density proximate the substrate is often significantly reduced since the substrate is generally downstream from the plasma source.
In general, known systems are not suitable for both generating a high density plasma and allowing appropriate control of the ion energy delivered to the substrate. Specifically, when, in the known systems, the plasma density is increased, the plasma potential is correspondingly raised and this leads to high sheath voltages and high ion energies in the vicinity of the substrate.
Furthermore, a growing problem associated with many PVD chambers is a lack of directivity for the adatom specie. Subsequently, when filling high aspect vias and trenches, the feature may be xe2x80x9cpinched offxe2x80x9d prematurely due to deposition coating growth on the feature side-walls. Therefore, the concept of xe2x80x9cion platingxe2x80x9d was introduced wherein a fraction of the adatom specie is ionized and attracted to the substrate surface by means of a substrate bias. Consequently, the directed (normal) flow of ionized adatom specie enabled a uniform coating growth without pinch-off; see U.S. Pat. No. 5,792,522.
In addition, an inherent problem associated with the use of inductively coupled plasma sources for conductive material PVD applications is the development of conductive coatings on the chamber walls. Once this thin coating exceeds some thickness (xcx9c400 A), the wall coating becomes sufficiently conductive that the plasma source ceases to sustain a plasma. In both U.S. Pat. Nos. 5,800,688 and 5,763,851, a circumferential shield comprising longitudinal and transverse structures is employed to limit the field of view of sputtered material migrating towards the chamber walls, hence, interrupting the generation of a continuous wall coating of conducting material. FIG. 1 of U.S. Pat. No. 5,800,688 shows a series of angular blade sections displaced circumferentially, whereas FIG. 2 U.S. Pat. No. 5,763,851 shows a series of concentric shields comprising longitudinal slots, with slots alternately placed between adjacent layers.
A primary object of the present invention is to form high quality films on a substrate.
Another object of the present invention to provide a method and apparatus for performing PVD in a plasma region which contains a high density plasma and in which the target is bombarded with high energy ions and the substrate is bombarded with comparatively low energy ions.
A further object of the invention is to allow independent control of the target and substrate bombardment ion energies.
A further object of the invention is to achieve high target material sputter rates in order to produce corresponding high film deposition rates.
Yet another object of the invention is to provide a novel target electrode structure which allows the desired high sputtering rate to be achieved.
Further objects of the invention are to reduce particle contamination due to complex geometric features in the process chamber, and to improve the efficiency of coupling RF power to the plasma.
Another object of the invention is to eliminate certain obstacles to the plasma-assisted deposition of conductive layers on substrates.
A still further object of the invention is to control the buildup of sputtered material on the chamber walls.
The above and other objects are achieved, according to the present invention, by apparatus for performing physical vapor deposition of a layer on a substrate, which apparatus includes a deposition chamber enclosing a plasma region for containing an ionizable gas, and electromagnetic field generating means surrounding the plasma region for inductively coupling an electromagnetic field into the plasma region to ionize the gas and generate and maintain a high density, low potential plasma. A source of deposition material including a solid target is installed in the chamber. The solid target contains a material which is to be deposited onto the substrate. The target is electrically biased in order to cause ions in the plasma to strike the target and sputter material from the target. The apparatus further includes a substrate holder for holding the substrate at a location to permit material sputtered from the target to be deposited on the substrate.
In preferred embodiments of the apparatus according to the invention, the electromagnetic field generating means comprise an electrostatically shielded radio frequency electromagnetic field source that includes an electrostatic shield surrounding the plasma region and a coil surrounding the shield for converting RF power into an electromagnetic field that is coupled into the plasma through the electrostatic shield.
It has been found that a source of this type is capable of generating a high density plasma, which makes possible the achievement of high target sputter rates and high substrate deposition rates while maintaining a low plasma potential and, hence, enabling independent control of the DC self-bias induced in each of the target and substrate.
Objects according to the invention are further achieved by the provision, in such apparatus, of a target assembly constructed to have the following features: the area of the target is small relative to the unbiased area of the chamber, thereby maximizing DC self-bias of the target; a match network is provided for permitting application of a high frequency RF voltage to create the self-bias, the target presenting a low impedance to high frequencies; the match network is constructed to minimize harmonics of the high frequency on the target; some RF power is capacitively coupled to the target; the target is brazed to a stainless steel plate which maximizes heat transfer from the target to a heat removal system; the heat removal system is constructed to be capable of high heat removal rates, typically in excess of 10 W/cm2; and a network for monitoring electrode arcing is provided.
Objects according to the invention are further achieved by the provision of a substrate holder in the form of a chuck having the following features: the chuck is constructed to electrostatically clamp the substrate to the chuck; a system for delivering gas to the lower surface of the substrate is provided; the chuck is constructed to remove heat at a high rate from the substrate; and a RF bias is applied to the chuck in order to induce a DC self-bias in the substrate. The combination of all four of these features will facilitate the delivery of a high ion flux to the substrate and, consequently, high deposition rates, while ions having a relatively low energy also impact on the substrate. As noted earlier herein, a certain amount of low energy in bombardment is needed to produce the highest quality depositing films.
In further accordance with the invention, the electrostatic shield of the electromagnetic field generating means is arranged to be electrically and/or thermally biased. The application of such bias during a sputtering process or between sputtering operations can serve to remove material that has been deposited on the inner surface of the shield, which constitutes a lateral wall of the deposition chamber. This at least reduces, if not completely eliminating, wall contamination and, thus, subsequent substrate contamination. The thickness of material deposited on the wall can be monitored, and this will provide an indication of when a wall cleaning is required.
Also in accordance with the invention, the apparatus may be configured to permit deposition of conductive material by providing a dielectric process tube that surrounds and delimits the plasma region and a bias shield that surrounds the process tube and is used for monitoring the thickness of a conductive material layer deposited on the process tube and to provide a bias voltage that acts to sputter the conductive material off of the process tube.